A wobble plate type compressor which reciprocates pistons by converting the rotational movement of a cam rotor into nutational movement of a wobble plate is well known in the art. A variable capacity mechanism which changes compression capacity is also well known, as shown in U.S. Pat. No. 3,861,829. In this mechanism, piston displacement is altered by varying the angle of the inclined surface of the cam rotor by a pressure difference between the crank chamber in which the cam rotor is disposed and the suction chamber. Thus, the compression capacity of the compressor varies with the piston displacement.
One of the disadvantages of the above mechanism is that the lower valve does not determine the suction pressure at which the variable capacity mechanism begins operating. Because the suction pressure of the refrigerant corresponds to the evaporating temperature of the refrigerant, if the lower valve determines the suction pressure, the surface of the evaporator may freeze. Thus, the pull-down characteristics of the compressor are not sufficient. Also, because the pressure in the crank chamber is controlled and the volume of the crank chamber is larger than that of the suction chamber, the piston response to changing the angle of the inclined surface of the cam rotor is not adequate. Furthermore, when the pressure difference between the crank chamber and the suction chamber changes, oil may flow into the crank chamber from the suction chamber.